観測井を利用した山崎断層の比抵抗連続観測

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The electrical well logging method has been applied for continuously measuring the earth resistivity. Both the current and the potential electrodes have been installed in an observation bore hole drilled very close to the Yamasaki fault in Hyogo prefecture. Electric currents of a rectangular wave form of an amplitude of 10 mA with a period of 10 second are being applied between the current electrodes and the apparent resistivity is computed from the potential difference measured between the adjacent potential electrodes. It was found that the apparent resistivity had increased about 10% between March and September, 1980. After that it gradually decreased. Superposed on this gradual change are small fluctuations closely related to changes in the water level in the bore hore. An examination of these short period fluctuations indicates that the apparent resistivity increases about 0.7 % for a one meter rise in water level with a time lag of 8 hours. On the other hand, the gradual increase of 10% in the resistivity observed between March and September is supposed to be a part of the seasonal variations. It was found that this change could be well correlated to the seasonal change in strain observed by an extensometer installed in the direction of N56°E in a tunnel across the Yamasaki fault about 10 m from the bore hole. Although the bore hole used for this experiment is only 20 m deep, the present technique will be applicable in deeper wells to measure the resistivity of the deeper crust.
1982-01-08

観測井を利用した山崎断層の比抵抗連続観測

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